Conventionally, a motor vehicle comprises an electronic control unit (also called BCM or Body Control Module) for controlling a certain number of items of equipment in the vehicle. Among this equipment mention may be made, for example, of the electrical control system for the windows of the vehicle, the electrical control system for the rear view mirrors, the air conditioning system, the vehicle immobilization system, the central locking system, etc. Such an electronic control unit generally comprises a microcontroller, a voltage regulator, a clock, inputs, outputs, a memory, etc. This electronic control unit is powered by the vehicle battery. The higher the operating frequency of the electronic control unit, the higher the power consumption.
The electronic control unit has two configuration modes:                a low-power mode during which the electronic control unit is on standby and has the main function of examining the periodically powered sources for awakening said electronic control unit from sleep mode;        a high-power mode, triggered immediately following detection of a source of awakening from sleep mode, such as, for example, a command to start the vehicle engine, and during which all the inputs of the electronic control unit are continuously powered, said electronic control unit thus being switched into a high-frequency operating mode via a phase locked loop.        
This variation in voltage has the following profile (illustrated in solid line in FIG. 1):                a first phase t1 referred to as the “initialize-start phase” corresponding to the switching-on of the vehicle engine starter. Because the starter draws a great deal of current, a significant drop in battery voltage is recorded during this phase, this voltage dropping from a nominal value UB, of the order of 13.5 Volts, to a value US of the order of 4.5 Volts;        an intermediate phase t2 referred to as the “on-starter phase” during which the battery voltage increases again and fluctuates around a value UD, these fluctuations corresponding to the action of the starter;        a last phase t3 referred to as the “exit-start phase” when the engine is turning over. A voltage rise is recorded during this phase, until the voltage returns to the nominal battery voltage value UB.        
When the battery is aging, or alternatively in cold weather with very low temperatures, there is a risk, during the voltage drop of the first phase t1, that the battery voltage will drop below an electronic control unit reset threshold USR and notably potentially lead to a loss of operation of said electronic control unit. This reset threshold USR varies between 3.5 and 4.5 Volts, depending on the characteristics of the electronic products.
It is known practice, in order to overcome this problem of resetting the electronic control unit, to use one or more capacitors, also referred to as “tank capacitors”, mounted in parallel, upstream of the voltage regulator, and making it possible to slow the drop in voltage of the first phase t1, preventing said voltage UC at the inlet of the electronic control unit from dropping below the reset threshold USR (as illustrated in dotted line in FIG. 1) and, therefore, ensuring that said electronic control unit maintains its functions. Thus, these tank capacitors, which are permanently charged, act as a buffer in case the battery should fail. The capacitance of these tank capacitors varies so as to ensure that the voltage at the output of the voltage regulator is above the reset threshold USR, and therefore depends on the consumption of the electronic control unit. Thus, the higher the consumption of the electronic control unit, the higher the capacitance of the tank capacitors. However, the higher the capacitance of the tank capacitors, the greater their size, which means that they occupy more space on the printed circuit of the electronic control unit, increasing the overall size thereof. In addition, these electronic components have an inevitable cost which it is appropriate to reduce.
It is also known and recommended practice to deactivate certain functions that are not essential to the starting phase such as, for example, the radiofrequency reception function, the functions responsible for authenticating the key providing access to the vehicle, for extinguishing lighting devices, wipers, etc., and to maintain, during the phase of starting the engine, only the essential functions, such as the communications on the CAN (Controller Area Network) bus, when the vehicle is equipped with such a data bus.
However, the current trend is toward providing vehicles that include ever increasing numbers of on-board functions, and this inevitably leads to an increase in the power consumption right from the phase of starting of the vehicle engine.